How do I know the number of polygons after adding a subdivision surface modifier without applying it?
This info, blatantly taken from the old manual page, gives you a clear idea on what to expect when subsurfing.



EDIT:
Unfortunately the link to the old manual is gone
http://wiki.blender.org/index.php/Doc:2.6/Manual/Modifiers/Generate/Subsurf
it was worth reading the whole page specially the Performance considerations.
From the new manual:

Performance Considerations
Higher levels of subdivisions mean more vertices, and more vertices means more memory will be used (both video memory for display, and system RAM for rendering). Blender could potentially crash or hang if you do not have enough memory.
When using high levels of subdivision with a graphics card that has a low total amount of Vram, some parts of the geometry will disappear visually. Your mesh will actually be OK, because the render is generated using your Object Data, (even though it cannot be shown by your graphics card).